FIG. 1 is a diagram illustrating a network structure of an E-UMTS (Evolved-Universal Mobile Telecommunications System). An E-UMTS is a system evolving from the conventional WCDMA (wideband code division multiple access) UMTS and its basic standardization is currently handled by the 3GPP (3rd Generation Partnership Project). The E-UMTS can also be called an LTE (Long Term Evolution) system.
Referring to FIG. 1, an E-UTRAN (UMTS terrestrial radio access network) includes base stations (hereinafter, referred to as ‘eNode B’ or ‘eNB’), wherein the respective eNBs are connected with each other through X2 interface. Also, each of eNBs is connected with a user equipment (UE) through a radio interface and connected with an EPC (Evolved Packet Core) through S1 interface. The EPC includes MME/SAE gateway (Mobility Management Entity/System Architecture Evolution gateway).
Layers of a radio interface protocol between a UE and a network can be classified into a first layer L1, a second layer L2 and a third layer L3 based on three lower layers of OSI (open system interconnection) standard model widely known in communication systems. A physical layer belonging to the first layer L1 provides an information transfer service using a physical channel. A radio resource control (hereinafter, abbreviated as ‘RRC’) layer located at the third layer plays a role in controlling radio resources between the UE and the network. For this, the RRC layer enables RRC messages to be exchanged between the UE and the network. The RRC layer may distributively be located at network nodes including Node B, an AG and the like, or may independently be located at either the Node B or the AG
FIG. 2 is a schematic view illustrating an E-UTRAN (UMTS terrestrial radio access network). In FIG. 2, a hatching part represents functional entities of a user plane and a non-hatching part represents functional entities of a control plane.
FIG. 3A and FIG. 3B illustrate a structure of a radio interface protocol between the user equipment (UE) and the E-UTRAN, in which FIG. 3A is a schematic view of a control plane protocol and FIG. 3B is a schematic view of a user plane protocol. Referring to FIG. 3A and FIG. 3B, a radio interface protocol horizontally includes a physical layer, a data link layer, and a network layer and vertically includes a user plane for data information transfer and a control plane for signaling transfer. The protocol layers in FIG. 3A and FIG. 3B can be classified into L1 (first layer), L2 (second layer), and L3 (third layer) based on three lower layers of the open system interconnection (OSI) standard model widely known in the communications systems.
The physical layer as the first layer provides information transfer service to an upper layer using physical channels. The physical layer is connected to a medium access control (hereinafter, abbreviated as ‘MAC’) layer above the physical layer via a transport channel. Data are transferred between the medium access control layer and the physical layer via the transport channel. Moreover, data are transferred between different physical layers, and more particularly, between one physical layer of a transmitting side and the other physical layer of a receiving side via the physical channel. The physical channel of the E-UMTS is modulated according to an orthogonal frequency division multiplexing (OFDM) scheme, and time and frequency are used as radio resources.
The medium access control (hereinafter, abbreviated as ‘MAC’) layer of the second layer provides a service to a radio link control (hereinafter, abbreviated as ‘RLC’) layer above the MAC layer via a logical channel. The RLC layer of the second layer supports reliable data transfer. In order to effectively transmit IP packets (e.g., IPv4 or IPv6) within a radio-communication period having a relatively narrow bandwidth, a PDCP layer of the second layer (L2) performs header compression to reduce unnecessary control information.
A radio resource control (hereinafter, abbreviated as ‘RRC’) layer located on a lowest part of the third layer is defined in the control plane only and is associated with configuration, reconfiguration and release of radio bearers (hereinafter, abbreviated as ‘RBs’) to be in charge of controlling the logical, transport and physical channels. In this case, the RB means a service provided by the second layer for the data transfer between the UE and the UTRAN.
Hereinafter, the PDCP layer included in the second layer will be described.
The PDCP layer is connected with its upper layer, i.e., RRC layer or a user application layer, and is connected with its lower layer, i.e., RLC layer. Main functions carried out by the PDCP layer include a header compression function and a security function. The header compression function is used to enhance use efficiency of radio resources, and is advantageous in that the quantity of information to be transmitted from a radio terminal is decreased using common features of packets transmitted through a single Internet packet stream. Examples of the security function include ciphering and integrity check, and the security function is used to prevent data manipulation or monitoring by the third party from occurring.
Of a header compression scheme used in the PDCP layer, a robust header compression (ROHC) scheme is used to reduce header information of RTP (Real-time Transport Protocol)/UDP (User Datagram Protocol)/IP (Internet Protocol) packets. In addition to the ROCH scheme, another example of the header compression scheme includes RFC2507.
The ROHC scheme is based on the fact that field values of packet headers in continuous packets belonging to one packet stream are almost constant. Accordingly, the ROHC scheme transmits a variable field not all the fields included in the packet headers. For reference, the entire header size of RTP/UDP/IP packets which are not compressed is 40 octet in case of IPv4 (IP version 4) and 60 octet in case of IPv6 (IP version 6) while size of a pure data part called payload is generally 15˜20 octet. Accordingly, it is noted that transmission efficiency is very low as control information has a structure greater than that of user data to be actually transmitted. In this respect, if the header compression scheme is used, the quantity of control information can be reduced remarkably. For example, size of the header reduced by the ROHC scheme is only 1 octet to 3 octets.
The ROHC scheme is divided into a uni-directional mode (hereinafter, abbreviated as ‘U-mode’), a bi-directional optimistic mode (hereinafter, abbreviated as ‘O-mode’) and a bi-directional reliable mode (hereinafter, abbreviated as ‘R-mode’). In case of the U-mode, the transmitting side performs uni-directional communication to the receiving side. In case of the O-mode or R-mode, the transmitting side performs bi-directional communication to transmit real-time packets and the receiving side transmits transmission status information to the transmitting side. Therefore, according to the ROHC scheme of the O-mode and the R-mode, the transmitting side transmits header compressed packets of data and controls transmission of real-time traffic packets in response to ROHC status information (ACK or NACK) received from the receiving side. A use purpose of the ROHC status information transferred from the receiving side to the transmitting side may depend on the mode. The ROHC scheme of the O-mode increases compression efficiency by mainly transmitting NACK related information, and the ROHC scheme of the R-mode supports robuster header compression scheme by using strict logic based on the ROHC status information. The ROHC status information can be called feedback information in a header compression procedure. In addition to the ROHC scheme, feedback information is also used in another header compression scheme.
Among the modes of the ROCH scheme, the U-mode will be described in detail. A compressor has three statuses, i.e., an entire context forming status, a dynamic context forming status, and an entire context perfect status. A type of a compression header packet is varied depending on each status, and its operation method is also varied depending on each status. First of all, a structure of a context will be described. The structure of a context includes a static context and a dynamic context.
FIG. 4 illustrates a status of an ROHC U-mode compressor according to the related art and its shift procedure. Referring to FIG. 4, the entire context forming status means that the entire context has not been formed at all, or the entire context should be reconstructed due to its damage. The dynamic context forming status means that a dynamic context part of the entire context should be reconstructed due to its damage. The entire context perfect status means that the entire context is in a perfect status without damage. Each status is shifted to another status per period. At this time, the respective periods are different from one another. For example, a shift period from the entire context perfect status to the dynamic context forming status is much greater than that from the entire context perfect status to the entire context forming status.